Bonded Tunable VCSEL with Bi-Directional Actuation

ABSTRACT

A MEMS tunable VCSEL includes a membrane device having a mirror and a distal-side electrostatic cavity for displacing the mirror to increase a size of an optical cavity. A VCSEL device includes an active region for amplifying light. Then, one or more proximal-side electrostatic cavities are defined between the VCSEL device and the membrane device and used to displace the mirror to decrease a size of an optical cavity.

BACKGROUND OF THE INVENTION

Optical coherence analysis relies on the use of the interferencephenomena between a reference wave and an experimental wave or betweentwo parts of an experimental wave to measure distances and thicknesses,and calculate indices of refraction of a sample. Optical CoherenceTomography (OCT) is one example of technology that is used to performhigh-resolution cross sectional imaging. It is often applied to imagingbiological tissue structures, for example, on microscopic scales in realtime. Optical waves are reflected from an object or sample and acomputer produces images of cross sections or three-dimensional volumerenderings of the sample by using information on how the waves arechanged upon reflection.

There are a number of different classes of OCT, but Fourier domain OCTcurrently offers the best performance for many applications. Moreover,of the Fourier domain approaches, swept-source OCT has distinctadvantages over techniques such as spectrum-encoded OCT because it iscompatible with balanced and polarization diversity detection. It alsohas advantages for imaging in wavelength regions where inexpensive andfast detector arrays, which are typically required for spectrum-encodedOCT, are not available.

In swept source OCT, the spectral components are not encoded by spatialseparation, but they are encoded in time. The spectrum is eitherfiltered or generated in successive optical frequency sampling intervalsand reconstructed before Fourier-transformation. Using the frequencyscanning swept source, the optical configuration becomes less complexbut the critical performance characteristics now reside in the sourceand especially its frequency sweep rate and tuning accuracy.

High speed frequency tuning, or high sweep rates, for OCT swept sourcesis especially relevant to in-vivo imaging where fast imaging reducesmotion-induced artifacts and reduces the length of the patientprocedure. It can also be used to improve resolution.

Historically, microelectromechanical systems (MEMS)-tunablevertical-cavity surface-emitting lasers (VCSELs) have been used intelecommunications applications. Their tunability enabled a single laserto cover multiple channels of the ITU wavelength division multiplexinggrid.

More recently, these MEMS tunable VCSELs have been proposed as the sweptsources in swept source OCT systems. Here, they have a number ofadvantages. Their short optical cavity lengths combined with the lowmass of their deflectable MEMS membrane mirrors enable high sweepspeeds. Moreover, they are capable of single longitudinal mode operationand are not necessarily subject to mode hopping noise. Thesecharacteristics also contribute to long coherence lengths for deepimaging.

In one example, a MEMS tunable VCSEL uses a VCSEL chip or device with anindium phosphide (InP)-based quantum-well active region with a galliumarsenide (GaAs)-based oxidized mirror. An electrostatically actuateddielectric mirror is suspended over the active region and separated byan air gap that forms part of the electrostatic cavity for thedielectric mirror. This electrostatically actuated mirror ismonolithically fabricated on top of the VCSEL device. The MEMS VCSEL isthen optically pumped by a 980 nanometer (nm) laser, for example.

Monolithically forming the MEMS dielectric mirror on the VCSEL creates anumber of disadvantages, however. First, any processes required to formMEMS mirror must be compatible with the chemistry of the VCSEL.Moreover, the complex fabrication sequence impacts manufacturing yields.

Another class of MEMS tunable VCSELs relies on bonding a MEMS mirrordevice to a VCSEL device. This allows for a separate electrostaticcavity, that is outside the laser's optical resonant cavity. Moreover,the use of this cavity configuration allows the MEMS mirror to be tunedby pulling the mirror away from the active region and the surface of theVCSEL device. This reduces the risk of snap down. Moreover, since theMEMS mirror device is now bonded to the VCSEL device, much widerlatitude is available in the technologies that are used to fabricate theMEMS mirror device. See for example U.S. Pat. No. 10,109,979 B2 to DaleC. Flanders, Mark E. Kuznetsov, Walid A. Atia and Bartley C. Johnson.

SUMMARY OF THE INVENTION

The present invention concerns MEMS tunable VCSELs. Different from priortunable VCSELs, however, the mirror can be pulled in the direction ofthe VCSEL device or optionally pulled away from that device. Moreover,in some of the embodiments and/or modes of operation, the mirror can bepulled in either direction in a dynamic fashion. In other cases, itmight be pulled to an initial position and then pulled further in thatdirection or pulled in the other direction.

Moreover, in some cases two proximal-side electrostatic cavities areprovided. These can be used to avoid the static charging of a mirror ofthe device. In addition or in the alternative, the two cavities can bedriven to improve the tuning performance of the device.

In general, according to one aspect, the invention features a tunablevertical cavity surface emitting laser (VCSEL), comprising a VCSELdevice including an active region for amplifying light and a membranedevice having a mirror and two proximal-side electrostatic cavitiesbetween the VCSEL device and the membrane device for displacing themirror to decrease a size of the optical cavity.

In embodiments, a first of the proximal-side electrostatic cavities aredefined between a membrane structure of the membrane device and aproximal-side electrostatic cavity electrode metal layer on the VCSELdevice. Then, a second of the proximal-side electrostatic cavities couldbe defined between the mirror of the membrane device and the VCSELdevice.

In some cases, a distal-side electrostatic cavity is also used fordisplacing the mirror to increase a size of an optical cavity. Adistal-side electrostatic cavity driver is useful for applying a voltageto the membrane device.

Preferably a first proximal-side electrostatic cavity driver is used forapplying a voltage across at least one of the two proximal-sideelectrostatic cavities. A second driver can also be employed.

In other cases, the mirror charging is avoided by placing the VCSELdevice at the same potential as the mirror.

In general, according to another aspect, the invention features a methodof tuning a vertical cavity surface emitting laser (VCSEL). It includesamplifying light in a VCSEL device including an active region anddriving at least one of two proximal-side electrostatic cavities betweenthe VCSEL device and a membrane device for displacing a mirror todecrease a size of the optical cavity.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is an exploded perspective view of a MEMS tunable VCSEL accordingto the present invention;

FIG. 2 is a front plan view showing the MEMS tunable VCSEL with theVCSEL device shown in phantom;

FIG. 3 is a side plan view showing the MEMS tunable VCSEL with the MEMSmirror device's optical port shown in phantom;

FIG. 4 is a front plan view showing the MEMS tunable VCSEL;

FIG. 5 is a cross-section taken along line A-A of FIG. 4 ;

FIG. 6 is a detailed cross-section taken along line B-B of FIG. 4 :

FIG. 7 is a plan view showing the VCSEL device;

FIG. 8 is a cross-section taken along line A-A and schematic showinganother embodiment of the VCSEL;

FIG. 9 is a cross-section taken along line A-A and schematic showingstill another embodiment of the VCSEL;

FIG. 10 is a cross-section taken along line A-A and schematic showingstill another embodiment of the VCSEL; and

FIG. 11 is a top plan view of an optically pumped tunable VCSEL sweptsource module including the gain embedded DBR VCSEL.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which illustrativeembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Further, the singular formsand the articles “a”, “an” and “the” are intended to include the pluralforms as well, unless expressly stated otherwise. It will be furtherunderstood that the terms: includes, comprises, including and/orcomprising, when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Further, it will be understood that when anelement, including component or subsystem, is referred to and/or shownas being connected or coupled to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

FIG. 1 shows a MEMS tunable VCSEL 100 comprising a MEMS membrane(mirror) device 110 that is bonded to an VCSEL chip or device 112, whichhas been constructed according to the principles of the presentinvention.

In the illustrated design, there is no separate spacer device separatingMEMS mirror device 110 from the VCSEL device 112. The general objectiveis to make the optical cavity of the tunable VCSEL 100 as small aspossible. Thus, in order to control the size of a free space portion ofthe optical cavity, various material layers are deposited on the MEMSmirror device 110 and/or the VCSEL device 112 in order to control thegap. This gap defines the free space portion, which extends between thesurface of the VCSEL device and the surface of the MEMS mirror device.In addition, according to the invention, at least one proximal-sideelectrostatic cavity also extends between the MEMS mirror device 110 andthe VCSEL device 112.

The optical membrane device 110 comprises handle wafer material 210 thatfunctions as a support. Currently, the handle is made from dopedsilicon, with resistivity <0.1 ohm-cm, carrier concentration >1×10¹⁷cm⁻³, to facilitate electrical contact.

An optical membrane or device layer 212 is added to the handle wafermaterial 210. A membrane structure 214 is formed in this opticalmembrane layer 212. In the current implementation, the membrane layer212 is silicon that is low doped with resistivity >1 ohm-cm, carrierconcentration <5×10¹⁵ cm⁻³, to minimize free carrier absorption of thetransmitted light. For electrical contact, the membrane layer surface isusually additionally doped with ion implantation to create a highlydoped surface layer. This method minimizes optical absorption in themembrane layer itself that would occur if the entire layer were highlydoped.

An insulating (buried silicon dioxide) layer 216 separates the opticalmembrane layer 212 from the handle wafer material 210.

Typically, silicon on isolator (SOI) wafers are used to provide thecombination of the handle wafer material 210, insulating oxide layer216, and the device layer 212.

During manufacture, the insulating layer 216 functions as asacrificial/release layer, which is partially removed to release themembrane structure 214 from the handle wafer material 210. Then duringoperation, the remaining portions of the insulating layer 216 provideelectrical isolation between the patterned device layer 212 and thehandle material 210.

In the current embodiment, the membrane structure 214 comprises a bodyportion 218. The optical axis of the device 100 passes concentricallythrough this body portion 218 and orthogonal to a plane defined by themembrane layer 212. A diameter of this body portion 218 is preferably300 to 600 micrometers; currently it is about 500 micrometers.

Tethers 220 (four tethers in the illustrated example) are defined byarcuate slots 225 fabricated into the device layer 212. The tethers 220extend radially from the body portion 218 to an outer portion 222, whichcomprises the ring where the tethers 220 terminate. In the currentembodiment, a spiral tether pattern is used.

A membrane mirror dot 250 is disposed on body portion 218 of themembrane structure 214. In some embodiments, the membrane mirror 250 isoptically curved to form an optically concave optical element to therebyform a curved mirror laser cavity. In other cases, the membrane mirror250 is a flat mirror, or even possibly convex.

When a curved membrane mirror 250 is desired, this curvature can becreated by forming a depression in the body portion 218 and thendepositing the material layer or layers that form mirror 250 over thatdepression. In other examples, the membrane mirror 250 can be depositedwith a high amount of compressive or tensile material stress or abackside AR coating 119 can be deposited with a high amount ofcompressive or tensile material that will result in its curvature.

The membrane mirror dot 250 is preferably a reflecting dielectric mirrorstack. In some examples, it is a dichroic mirror-filter that provides adefined reflectivity, such as between 1 and 10%, to the wavelengths oflaser light generated in the laser 100, whereas the optical dot 250 istransmissive to wavelengths of light that are used to optically pump theactive region in the VCSEL device 112. In still other examples, theoptical dot is a reflective metal layer such as aluminum or gold.

In the illustrated embodiment, three metal bond pads 234T, 234R, 234Lare deposited on the proximal side of the membrane device 110. These areused to solder or thermocompression bond, for example, the VCSEL device112 onto the proximal face of the membrane device 110. The top pad 234Talso provides an electrical connection to the VCSEL device 112.

Also provided are three wire bondpads 334A, 334B, and 334C. The leftVCSEL electrode wire bond pad 334A is used to provide an electricalconnection to the metal pad 234T. On the other hand, the right membranewire bond pad 334B is used to provide an electrical connection to themembrane layer 212 and thus the membrane structure 214. Finally, thehandle wire bond pad 334C is used to provide an electrical connection tothe handle wafer material 210.

The VCSEL device 112 generally comprises an antireflective coating 114,which is optional, and an active region 118, which preferably has asingle or multiple quantum well structure. The cap layer can be usedbetween the antireflective coating 114, if present, and the activeregion 118. The cap layer protects the active region from thesurface/interface effects at the interface to the AR coating and/or air.The back mirror 116 of the laser cavity is defined by a distributedBragg reflector (DBR) mirror. Finally, a VCSEL spacer 115, such as GaAS,functions as a substrate and mechanical support.

The material system of the active region 118 of the VCSEL device 112 isselected based on the desired spectral operating range. Common materialsystems are based on III-V semiconductor materials, including binarymaterials, such as GaN, GaAs, InP, GaSb, InAs, as well as ternary,quaternary, and pentenary alloys, such as InGaN, InAlGaN, InGaP, AlGaAs,InGaAs, GaInNAs, GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb,AlAsSb, InGaSb, InAsSb, and InGaAsSb. Collectively, these materialsystems support operating wavelengths from about 400 nanometers (nm) to2000 nm, including longer wavelength ranges extending into multiplemicrometer wavelengths. Semiconductor quantum well and quantum dot gainregions are typically used to obtain especially wide gain and spectralemission bandwidths.

In the preferred embodiment, the polarization of the light generated bythe MEMS tunable VCSEL 100 is preferably controlled or at leaststabilized. In general, this class of devices has a cylindricalresonator that emits linearly polarized light. Typically, the light ispolarized along the crystal directions with one of those directionstypically being stronger than the other. At the same time, the directionof polarization can change with laser current or pumping levels, and thebehaviors often exhibit hysteresis.

Different approaches can be taken to control the polarization. In oneembodiment, polarization selective mirrors are used. In another example,non-cylindrical resonators are used. In still a further embodiment,asymmetrical current injection is used when electrical pumping isemployed. In still other examples, the active region substrate includestrenches or materials layers, which result in an asymmetric stress,strain, heat flux or optical energy distribution, are used in order tostabilize the polarization along a specified stable polarization axis.In still a further example, asymmetric mechanical stress is applied tothe VCSEL device 112 as described in “Tunable VCSEL polarization controlthrough dissimilar die bonding” by Bartley C. Johnson, et al. U.S.patent application Ser. No. 16/409,295, filed on May 10, 2019, now U.S.Pat. No. 10,951,009, which is incorporated herein by this reference,hereinafter Johnson.

Defining the other end of the laser cavity is the rear mirror 116 thatis formed in the VCSEL device 112. In one example, this is a layeradjacent to the active region 116 that creates the refractive indexdiscontinuity that provides for a portion of the light to be reflectedback into the cavity, such as between one and 10%. In other examples,the rear mirror 116 is a high reflecting layer that reflects over 90% ofthe light back into the laser cavity.

In still other examples, the rear VCSEL distributed Bragg reflector(DBR) mirror 116 is a dichroic mirror-filter that provides a definedreflectivity, such as between 1 and 100%, to the wavelengths of laserlight generated in the laser 100, whereas the rear mirror 116 istransmissive to wavelengths of light that are used to optically pump theactive region in the VCSEL device 112, thus allowing the VCSEL device112 to function as an input port of pump light.

FIG. 2 is front view showing the MEMS tunable VCSEL 100 with the VCSELdevice 112 shown in phantom.

Notably, the view shows the arrangement of the VCSEL device bond pads120A-120E that are arrayed in an arc on the proximal side of the VCSELdevice 112 to enable it to be bonded to the bond pads 234T, 234R, 234Lof the optical membrane device 110.

FIG. 3 shows the MEMS tunable VCSEL 100 in side cross-section.

An optical port 240 is provided, extending from a distal side of thehandle wafer material 210 to the membrane structure in cases where themirror 250 is used as an output reflector or to provide for monitoring.If the reflector 250 is used as a back reflector, then the port 240 isnot necessary in some cases.

Further, whether or not this optical port 240 is required also dependsupon the transmissivity of the handle wafer material 210 at the opticalwavelengths over which the MEMS tunable VCSEL 100 must operate.Typically, with no port, the handle wafer material 210 along the opticalaxis must be anti-reflection coated (AR) coated if transmission throughthe backside is required for functionality.

FIG. 4 is front view showing the MEMS tunable VCSEL 100 showing sectionlines A-A and B-B.

FIG. 5 schematically shows the MEMS tunable VCSEL 100 in cross-sectionalong A-A to show a proximal-side electrostatic cavity and a distal-sideelectrostatic cavity 224.

The optical port 240 has generally inward sloping sidewalls 244 that endin the port opening 246. As a result, looking through the distal side ofthe handle wafer 210, the body portion 218 of the membrane structure 214is observed. The port is preferably concentric with the membrane mirrordot 250. Further, the backside of the body portion 218 is coated with amembrane backside AR coating 119 in some examples. This AR coating 119is used to facilitate the coupling of pump light into the laser cavityand/or the coupling of laser light out of the cavity. In still otherexamples, it is reflective to pump light to return pump light back intothe laser cavity.

The thickness of insulating layer 216 defines the electrostatic cavitylength of the distal-side electrostatic cavity 224. Presently, theinsulating layer 216 is between 1.5 and 6.0 μm thick. It is a generalrule of thumb, that electrostatic elements can be tuned over no greaterthan one third the distance of the electrostatic cavity. As a result,the body portion 218, and thus the mirror optical coating 230 can bedeflected between 1 and 3 μm in the distal direction (i.e., away fromthe VCSEL device 112), in one embodiment.

Also shown are details concerning how the VCSEL device 112 is bonded tothe membrane device 110. The MEMS device bond pads 234T, 234R, 234L bondto VCSEL proximal-side electrostatic cavity electrode metal 122. Thesemetal layers are electrically isolated. Specifically, the MEMS devicebond pads 234 are separated from the membrane layer 212 by MEMS devicebond pad isolation oxide layer 236; the VCSEL proximal-sideelectrostatic cavity electrode metal 122 is isolated from the remainderof the VCSEL device by VCSEL isolation oxide layer 128. Neither of theVCSEL proximal-side electrostatic cavity electrode metal 122 nor theVCSEL isolation oxide layer 128 interferes with the optical operationsince they do not extend into the region of the free-space portion 252of the laser's optical cavity.

The distal-side electrostatic cavity 224 and the proximal-sideelectrostatic cavities 226 are located on either side of the membranestructure 214. Specifically, the distal-side electrostatic cavity 224 iscreated between the handle wafer material 210 and the membrane structure214, which is the suspended portion of the membrane layer 212. A voltagepotential between the handle wafer material 210 and the membrane layer212 will generate an electrostatic attraction between the layers andpull the membrane structure 214 toward the handle wafer material 210. Onthe other hand, one or more proximal-side electrostatic cavities 226 arecreated between the membrane structure 214 and the VCSEL proximal-sideelectrostatic cavity electrode metal 122 and the VCSEL device 112generally. Potentially different voltage potentials between the membranelayer 212 and the VCSEL proximal-side electrostatic cavity electrodemetal 122 and the VCSEL device 112 will generate an electrostaticattraction between the layers and pull the membrane structure 214 towardthe VCSEL device 112.

In general, the size of at least one of the proximal-side electrostaticcavities 226 measured along the device's optical axis is defined by thebond metal thickness, thickness of VCSEL proximal-side electrostaticcavity electrode metal 122 and MEMS device bond pads 234 along with thethicknesses VCSEL isolation oxide layer 128 and MEMS device bond padisolation oxide 236.

The minimum oxide thickness is determined by the required voltageisolation. Oxide break down is nominally 1000V/micrometer. So, for 200Visolation that would be 2000 A, which is preferably doubled for margin.So the thickness of layers of VCSEL isolation oxide layer 128 and MEMSdevice bond pad isolation oxide 236 is greater than 4000 A.

The current metal bond thickness is 6000 A (each layer) with approx.3000 A compression during bonding. Based on this, the minimum size ofthe proximal-side electrostatic cavity 226 is 0.85 micrometers.

At this minimum electrostatic gap point, a zero optical gap results whenthe membrane mirror dot 250 is 1.7 micrometers thick.

To increase the optical gap, the thickness of the VCSEL isolation oxidelayer 128 can be increased without affecting the operation of thecavity.

In one embodiment, the layer thicknesses of VCSEL antireflective coating114, VCSEL proximal-side electrostatic cavity electrode metal 122, MEMSdevice bond pads 234, and MEMS device bond pad isolation oxide 236 andfor the HR coating (250) are such that, under conditions of electricaloverstress as the deflectable membrane structure (214) is pulled towardsthe VCSEL device (112), the surface of the membrane mirror dot 250 willtouch the surface of the VCSEL device 112 before the membrane structure214 can come into contact with the VCSEL proximal side electrode metal122. The contact of the membrane to the highly conductive VCSELelectrode metal can cause permanent electrical damage to the device,whereas the membrane mirror dot 250 is an insulator. This featureprotects the device against damage from such electrical overstress.

On the other hand, isolation oxide layer 128 is not necessary. In fact,if the VCSEL device is not isolated then the active area is also chargedthe same as the metal electrode. Since the HR coating 250 stack is adielectric, the equivalent to an air gap from the membrane to the VCSELis less. This appears to give a significant kick in electrostatic forceas the membrane and HR stack is pulled in.

FIG. 6 is a cross-section along B-B and shows a portion of the membranedevice 110 in the region of the handle wire bond pad 334C.

The handle wire bond pad 334C is fabricated by forming a hole 345through the membrane layer 112 and another hole 342 through the buriedoxide insulating layer 216. This exposes the handle wafer material 210,on which the handle wire bond pad 334C is deposited.

FIG. 7 shows the metal pattern on the proximal side of the VCSEL device112.

In some examples, only 4 pads are used however. The top pad 120C iseliminated to provide a preferential stress direction for polarizationcontrol as described in Johnson.

The VCSEL proximal-side electrostatic cavity electrode metal 122 coversthe center portion of the proximal side of the VCSEL device 112, but forthe very center, wherein the VCSEL antireflective coating 114 remainsexposed.

The VCSEL proximal-side electrostatic cavity electrode metal 122 iselectrically connected to VCSEL device bond pads 120B-120D by respectiveVCSEL bond pad-electrode bridges 124B-124D.

When assembled, the VCSEL proximal-side electrostatic cavity electrodemetal 122 is electrically connected to the VCSEL electrode wire bondpad334A by the metal bond between the VCSEL device bond pads 120B, 120C,120D and MEMS device bond pads 234L, 234T, 234R, see also FIG. 2 . TheMEMS device bond pads 234 in turn are electrically connected to theVCSEL electrode wire bondpad 334A by the VCSEL bridge metal 340.

Thus, with reference to FIG. 2 , a distal-side electrostatic cavitydriver 424 applies a voltage between the handle wafer material 210 viathe handle wire bond pad 334C and the membrane layer 212 via themembrane wire bond pad 334B. A proximal-side electrostatic cavity driver426 applies a voltage between the membrane layer 212 via the membranewire bond pad 334B and the VCSEL 112 or specifically the VCSELproximal-side electrostatic cavity electrode metal 122 via the leftVCSEL electrode wire bond pad 334A. In this way, a controller 400controls the proximal-side electrostatic cavity 226 by controlling theproximal-side electrostatic cavity driver 426 to translate the membranestructure 214 of the membrane layer 212 toward the VCSEL device 112, andcontroller 400 controls the distal-side electrostatic cavity 224 bycontrolling distal side electrostatic driver 424 to translate themembrane structure 214 of the membrane layer 212 toward the handlematerial 210.

FIG. 8 shows another embodiment of the MEMS tunable VCSEL 100 incross-section along A-A that has an outer proximal-side electrostaticcavity 226A and an inner proximal-side electrostatic cavity 226B withrespect to the center optical axis. In this embodiment, a metal contact130 is added to the backside of the VCSEL spacer 115.

An outer proximal-side electrostatic cavity driver 426A is connected asdescribed previously to apply a voltage between the membrane layer 212via the membrane wire bond pad 334B and the VCSEL proximal-sideelectrostatic cavity electrode metal 122 via the left VCSEL electrodewire bond pad 334A. In this way, the controller 400 controls an outerproximal-side electrostatic cavity 226A by controlling the outerproximal-side electrostatic cavity driver 426A to establish anelectrical potential or voltage to electrostatically translate themembrane structure 214 of the membrane layer 212 toward the VCSEL device112.

The controller 400 however also controls an inner proximal-sideelectrostatic cavity 226B by controlling inner proximal-sideelectrostatic driver 426B to generate an electrostatic force to furthertranslate the membrane structure 214 of the membrane layer 212 towardthe VCSEL device 112. Specifically, the inner proximal-sideelectrostatic cavity driver 426B is connected to both the membrane wirebond pad 334B and the metal VCSEL spacer contact 130 to apply anelectrical potential or voltage between the VCSEL device 112 andmembrane layer 212 under the control of the controller 400. Thisincreases the pull-in force by effectively increasing the area of theelectrostatic cavity on the proximal side. In addition, the gap in theinner proximal-side electrostatic cavity is often smaller in absoluteterms in the direction of the optical axis, and tends to have higherdimensional precision in terms of manufacturing variability. Moreover,the presence of the membrane mirror 250 increases the electric fieldstrength, the electrical permittivity of the mirror effectively reducesthe gap.

In one mode of operation, one of the drivers: the outer proximal-sideelectrostatic cavity driver 426A or the inner proximal-sideelectrostatic driver 426B, is used to pull the membrane structure to aninitial position associated with a desired start emission wavelength ofthe VCSEL 100, then the other of the two drivers 426A, 426B is used bythe controller to sweep the emission wavelength through a desiredspectral scan band.

FIG. 9 shows another embodiment of the MEMS tunable VCSEL 100 incross-section along A-A that has an outer proximal-side electrostaticcavity 226A and an inner proximal-side electrostatic cavity 226B. Inthis embodiment, the metal contact 130 is again added to the backside ofthe VCSEL spacer 115.

The outer proximal-side electrostatic cavity driver 426A is connected toapply a voltage between the membrane layer 212 via the membrane wirebond pad 334B and the VCSEL proximal-side electrostatic cavity electrodemetal 122 via the left VCSEL electrode wire bond pad 334A. In this way,the controller 400 controls the outer proximal-side electrostatic cavity226A by controlling the outer proximal-side electrostatic cavity driver426A to translate the membrane structure 214 of the membrane layer 212toward the VCSEL device 112.

The controller 400 however also controls the inner proximal-sideelectrostatic cavity 226B by controlling the inner proximal-sideelectrostatic driver 426B to further translate the membrane structure214 of the membrane layer 212 toward the VCSEL device 112. In thisexample, the inner proximal-side electrostatic cavity driver 426B isconnected to both the VCSEL proximal-side electrostatic cavity electrodemetal 122 via the left VCSEL electrode wire bond pad 334A and the metalVCSEL spacer contact 130 to apply a separately controlled voltagebetween the VCSEL device 112 and membrane layer 212, again increasingthe pull-in force by effectively increasing the area of theelectrostatic cavity on the proximal side.

In this embodiment, the distal-side electrostatic cavity driver 424 canalso be used to apply a voltage between the handle wafer material 210via the handle wire bond pad 334C and the membrane layer 212 via themembrane wire bond pad 334B to additionally pull the membrane structure214 toward the handle wafer 210.

FIG. 10 shows another embodiment of the MEMS tunable VCSEL 100 incross-section along A-A that is configured to avoid charging of thedielectric layers of the mirror 250.

Here a proximal-side electrostatic cavity driver 426 applies a voltagebetween the VCSEL proximal-side electrostatic cavity electrode metal 122via the left VCSEL electrode wire bond pad 334A and both the metalcontact 130 of the VCSEL spacer 115 and the membrane wire bond pad 334B.This way, the VCSEL 112 and the membrane layer 212 are at the samepotential during operation and the inner proximal-side electrostaticcavity 226B is basically deactivated as a driving modality. Thus, onlythe outer proximal-side electrostatic cavity 226A is used to deflect themembrane structure 214 toward the VCSEL. The distal-side electrostaticcavity driver 424 is used to apply a voltage between the handle wafermaterial 210 via the handle wire bond pad 334C and the membrane layer212 via the membrane wire bond pad 334B to additionally pull themembrane structure 214 toward the handle wafer 210 under the operationof the controller 400.

FIG. 11 also shows an example of an optically pumped tunable VCSEL sweptsource system 101 employing the VCSEL 100, which system has beenintegrated into a single module.

Light from a pump chip 760 is coupled to a bench 740 via a pump opticalfiber 742. The pump light 712 from the optical fiber 742 is collimatedby a first lens LensA that is affixed to the bench 740. The pump light712 then is transmitted through the dichroic mirror 732 and then focusedby a second lens LensB onto the half VCSEL 112 of the VCSEL 100.

Preferably, the bench 740, in turn, is installed in a hermetic package744 with optical fibers passing through fiber-feedthroughs 746, 748 ofthe package 744.

The dichroic mirror 732 is reflective to longer wavelength of the VCSELlight 734, emitted by the VCSEL 100, but transmissive to the pump light712, 724 in the illustrated example. Specifically in the illustratedexample, the tunable signal from the VCSEL 100 is reflected by thedichroic mirror 732, which is affixed to the bench 740, and directed toa fold mirror 750 which is also affixed to the bench 740 and then to athird lens 752, which is affixed to the bench 740. The third lens 752focuses light into an entrance aperture of an output optical fiber 754.

More details of this specific design can be found in U.S. Pat. Appl.Pub. No. US 2019/0348813 A1, which is incorporated herein by thisreference in its entirety.

During operation, the controller 400 energizes the pump 760 to opticallypump active region 118 of the VCSEL device 112. At the same time, thecontroller 400 controls the proximal-side electrostatic cavity 226 andcontroller 400 controls the distal-side electrostatic cavity 224 bycontrolling distal side electrostatic driver 424.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A tunable vertical cavity surface emitting laser (VCSEL), comprising: a VCSEL device including an active region for amplifying light; and a membrane device having a mirror and two proximal-side electrostatic cavities between the VCSEL device and the membrane device for displacing the mirror to decrease a size of the optical cavity.
 2. The VCSEL as claimed in claim 1, wherein a first of the proximal-side electrostatic cavities are defined between a membrane structure of the membrane device and a proximal-side electrostatic cavity electrode metal layer on the VCSEL device.
 3. The VCSEL as claimed in claim 1, wherein a second of the proximal-side electrostatic cavities are defined between the mirror of the membrane device and the VCSEL device.
 4. The VCSEL as claimed in claim 1, further comprising a distal-side electrostatic cavity for displacing the mirror to increase a size of an optical cavity.
 5. The VCSEL as claimed in claim 4, further comprising a distal-side electrostatic cavity driver for applying a voltage to the membrane device.
 6. The VCSEL as claimed in claim 1, further comprising a proximal-side electrostatic cavity driver for applying a voltage across at least one of the two proximal-side electrostatic cavities.
 7. The VCSEL as claimed in claim 1, further comprising two proximal-side electrostatic cavity drivers for applying separate voltages across each of the two proximal-side electrostatic cavities.
 8. The VCSEL as claimed in claim 1, further comprising avoiding charging of the mirror by placing the VCSEL device at the same potential as the mirror.
 9. The VCSEL as claimed in claim 1, wherein the VCSEL is protected against damage due to electrical overstress of the proximal-side electrostatic cavities by ensuring that the gap in the electrostatic cavity is prevented from going to 0 by use of an insulating stand-off.
 10. The VCSEL as claimed in claim 6, wherein the insulating stand off is the high reflective dielectric coating.
 11. A method of tuning a vertical cavity surface emitting laser (VCSEL), comprising: Amplifying light in a VCSEL device including an active region; and driving at least one of two proximal-side electrostatic cavities between the VCSEL device and a membrane device for displacing a mirror to decrease a size of the optical cavity.
 12. The method as claimed in claim 11, wherein a first of the proximal-side electrostatic cavities are defined between a membrane structure of the membrane device and a proximal-side electrostatic cavity electrode metal layer on the VCSEL device.
 13. The method as claimed in claim 12, wherein a second of the proximal-side electrostatic cavities are defined between the mirror of the membrane device and the VCSEL device.
 14. The method as claimed in claim 1, further comprising driving a distal-side electrostatic cavity for displacing the mirror to increase a size of an optical cavity.
 15. The method as claimed in claim 14, further comprising a distal-side electrostatic cavity driver for applying a voltage to the membrane device.
 16. A tunable vertical cavity surface emitting laser (VCSEL), comprising: a VCSEL device including an active region for amplifying light; and a membrane device having a displaceable mirror for decreasing a size of the optical cavity; and an electrical contact to the VCSEL device for controlling a potential between the VCSEL device and the membrane device in a region of the displaceable mirror. 